Span to tonnage, because steel is bought and quoted by mass, not by length.
Foundation, roof and structure. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Steel I-Beam Calculator tool.
A steel beam is bought by weight, so the calculation is a section multiplied by a length. Everything that makes it interesting sits around that multiplication: which profile, what mass per metre, how long, and what happens when the span gets long enough that the section stops being the only question. The estimator returns the weight and the cost from a price you enter, because there is no honest default for steel in a market without a verified price book. That is a deliberate refusal, not a gap. Where the tool has a verified price it uses it; where it does not, it asks, because a made up tonne rate is the kind of number that ends up in a tender and then in an argument.
Two beams of six metres in a three hundred by one hundred and fifty universal beam, at a mass of about forty kilogrammes per metre, come to four hundred and eighty kilogrammes plus a five percent cutting allowance, which is five hundred and four. Enter a price of one thousand per tonne and the steel is five hundred and four units of cost, with fabrication, connections and fire protection outside it. Push the same beam to nine metres and the page raises the span flag, because beyond the threshold the section is no longer something to select from a table without a check.
Multiply the mass per metre of the profile by the length, then by the number of members. The profile's mass is a published property, so a three hundred by one hundred and fifty universal beam carries roughly forty kilogrammes for every metre of its length, and two six metre beams weigh four hundred and eighty kilogrammes before any cutting allowance. The calculator does that from the profile and length you select, and summarises the order by profile so a fabricator can quote from it.
There is no single answer, because the span depends on the load above it, the restraint the floor gives the top flange and the deflection the finish can tolerate. What the page does is flag the point at which a section stops being safely selectable from a table, at eight metres on the default rule, and say so rather than return a section. Beyond that point the span is an engineering question with a real answer, and it is not one a weight calculator should invent.
Because there is no verified steel price book for most markets. A made up rate per tonne would produce a total that looks authoritative and is wrong, and a quotation built on it fails when it meets a real supplier. The tool asks for the price you have been quoted and multiplies it, so every cost on the page can be traced to a document. Where a market does have a verified price, the tool uses it.
From the design, and the tool measures what the design specifies. Depth follows the span and the load, mass follows the depth once the profile family is chosen, and the family itself follows the connection detail and what the fabricator stocks. Where two profiles work structurally, the lighter one is usually the cheaper one to erect and the heavier one the cheaper to buy, which is a commercial judgement rather than a calculation.
No. Connections, base plates, bolts, welds and fire protection are all outside the estimate, as are erection, crane hire and painting. They are priced by the fabricator and the erector, and they are often a third of the delivered cost of the steel. The estimate covers the members by profile and weight, which is the part a supplier quotes on and the part you can check with a tape measure on delivery.